Electron spin relaxation can enhance the performance of a cryptochrome-based magnetic compass sensor

The radical pair model of the avian magnetoreceptor relies on long‐lived electron spin coherence. Dephasing, resulting from interactions of the spins with their fluctuating environment, is generally assumed to degrade the sensitivity of this compass to the direction of the Earth's magnetic field. Here we argue that certain spin relaxation mechanisms can enhance its performance. We focus on the flavin‐tryptophan radical pair in cryptochrome, currently the only candidate magnetoreceptor molecule. Correlation functions for fluctuations in the distance between the two radicals in Arabidopsis thaliana cryptochrome 1 were obtained from molecular dynamics simulations and used to calculate the spin relaxation caused by modulation of the exchange and dipolar interactions. We find that intermediate spin relaxation rates afford substantial enhancements in the sensitivity of the reaction yields to an Earth‐strength magnetic field. Supported by calculations using toy radical pair models, we argue that these enhancements could be consistent with the molecular dynamics and magnetic interactions in avian cryptochromes

Charge-state dependent vibrational relaxation in a single-molecule junction

The interplay between nuclear and electronic degrees of freedom strongly influences molecular charge transport. Herein, we report on transport through a porphyrin dimer molecule, weakly coupled to graphene electrodes, that displays sequential tunneling within the Coulomb-blockade regime. The sequential transport is initiated by current-induced phonon absorption and proceeds by rapid sequential transport via a non-equilibrium vibrational distribution. We demonstrate this is possible only when the vibrational dissipation is slow relative to sequential tunneling rates, and obtain a lower bound for the vibrational relaxation time of 8 ns, a value that is dependent on the molecular charge state.Comment: 8 pages, 7 figure

Distinguishing Lead and Molecule States in Graphene-Based Single-Electron Transistors

Graphene provides a two-dimensional platform for contacting individual molecules, which enables transport spectroscopy of molecular orbital, spin, and vibrational states. Here we report single-electron tunneling through a molecule that has been anchored to two graphene leads. Quantum interference within the graphene leads gives rise to an energy-dependent transmission and fluctuations in the sequential tunnel-rates. The lead states are electrostatically tuned by a global back-gate, resulting in a distinct pattern of varying intensity in the measured conductance maps. This pattern could potentially obscure transport features that are intrinsic to the molecule under investigation. Using ensemble averaged magneto-conductance measurements, lead and molecule states are disentangled, enabling spectroscopic investigation of the single molecule

Vibrational effects in charge transport through a molecular double quantum dot

Recent progress in the field of molecular electronics has revealed the fundamental importance of the coupling between the electronic degrees of freedom and specific vibrational modes. Considering the examples of a molecular dimer and a carbon nanotube double quantum dot, we here theoretically investigate transport through a two-site system that is strongly coupled to a single vibrational mode. Using a quantum master equation approach, we demonstrate that, depending on the relative positions of the two dots, electron-phonon interactions can lead to negative differential conductance and suppression of the current through the system. We also discuss the experimental relevance of the presented results and possible implementations of the studied system

Vibrational effects in quantum transport through single-molecule junctions

This thesis is concerned with vibrational effects in resonant charge transport through molecular junctions. The first research chapter examines the role of non-equilibrium vibrational dynamics in charge transport through a molecular double quantum dot. We demonstrate that non-equilibrium vibrational effects in such a system can result in current suppression and negative differential conductance. The second research chapter describes how vibrational dynamics can be incorporated into the theoretical description of molecular junctions based on a Pariser-Parr-Pople Hamiltonian. As a case study, we consider a prototypical spiro-conjugated molecular junction. We demonstrate that the transport properties of this system are governed by an interplay between destructive quantum interference and Jahn-Teller distortion (an effect of vibronic origin). In the next chapter, we examine the role of collective vibrational coupling in charge transport through a two-site molecular system akin to the one considered in the first research chapter. We demonstrate that such interactions can significantly enhance the efficiency of transport through such a system, and also take this opportunity to analyse various theoretical descriptions of electron-vibrational interactions commonly used in the transport setting. In the fourth research chapter, we consider a single-site molecular system coupled to a collection of thermalised vibrational modes. We formulate a relatively simple yet powerful theoretical framework describing charge transport through such a system. We recover the Marcus and Landauer-Buttiker theories of transport in the limit of high-temperature and vanishing electron-vibrational coupling, respectively. We further show how lifetime broadening can be consistently incorporated into Marcus theory, and we derive a low-temperature correction to the semi-classical Marcus hopping rates. In the last research chapter, elements of this framework are applied to experimental measurements of charge transport through zinc-porphyrin molecular junctions. This joint experimental-theoretical study provides, for the first time, a full understanding of the resonant transport regime of graphene-based molecular junctions.</p

Beyond Marcus theory and the Landauer–Büttiker approach in molecular junctions. II. A self-consistent Born approach

Marcus and Landauer-Buttiker approaches to charge transport through molecular junctions describe two contrasting mechanisms of electronic conduction. In previous work, we have shown how these charge transport theories can be unified in the single-level case by incorporating lifetime broadening into the second-order quantum master equation. Here, we extend our previous treatment by incorporating lifetime broadening in the spirit of the self-consistent Born approximation. By comparing both theories to numerically converged hierarchical-equations-of-motion (HEOM) results, we demonstrate that our novel self-consistent approach rectifies shortcomings of our earlier framework which are present especially in the case of relatively strong electron-vibrational coupling. We also discuss circumstances under which the theory developed here simplifies to the generalised theory developed in our earlier work. Finally, by considering the high-temperature limit of our new self-consistent treatment, we show how lifetime broadening can also be self-consistently incorporated into Marcus theory. Overall, we demonstrate that the self-consistent approach constitutes a more accurate description of molecular conduction while retaining most of the conceptual simplicity of our earlier framework.Comment: Version accepted in J. Chem. Phy

Role of metallic leads and electronic degeneracies in thermoelectric power generation in quantum dots

The power factor of a thermoelectric device is a measure of the heat-to-energy conversion efficiency in nanoscopic devices. Yet, even as interest in low-dimensional thermoelectric materials has increased, experimental research on what influences the power factor in these systems is scarce. Here, we present a detailed thermoelectric study of graphene quantum dot devices. We show that spin degeneracy of the quantum dot states has a significant impact on the zero-bias conductance of the device and leads to an increase of the power factor. Conversely, we demonstrate that nonideal heat exchange within the leads can suppress the power factor near the charge degeneracy point and nontrivially influences its temperature dependence.QN/van der Zant La